I dove in a similar cavern (Cenote). The "cloud layer" you see is the result of decomposing organic matter (leaves, etc.) that release a sulfur-based gas. The bottom is completely saturated with it (high specific density), and on top of if there's a layer of fresh water that keeps it "capped" (with low specific density). This depends on the water not being mixed (no currents). The funny thing is that the "clouds" are only in the transition layer. Once you dive below it, the water is clear again. The transition is so sharp that when coming out of this cloud layer, it is similar to the transition between water and air; we were half-jokingly warned not to make this mistake (trying to breathe this water is not recommended :-). It was like flying inside some creepy horror movie scene - one of the most memorable dives I had.And no, it isn't toxic or acidic or anything. Both my gear and myself are fine (and I went under the "cloud layer" into the clear saturated water below).